1. Field of the Invention
The present invention relates to a joint structure for joining a branch pipe, branch adapter or similar members to a high pressure fuel rail such as a high pressure fuel multi-pipe and a block rail and in particular, to a joint structure for joining a branch member to a high pressure fuel rail under a pressure of greater than 1000 kgf/cm.sup.2 such as a fuel supply line for use in a diesel internal combustion engine.
2. Description of the Prior Art
A conventional joint structure of this type is disclosed in Japanese laid-open patent publication No. 3-273599 filed by the applicants of this application. Referring specifically to FIG. 11, a branch member or pipe 13 includes an internal passage 13". The branch pipe 13 has a spigot head 13' at its distal end. The spigot head 13' has a spherical surface 14'. The branch pipe 13 has a straight portion (cylindrical wall) 14 provided at its proximal end and connected to the spherical surface 14'. The spigot head 13' has a radius of curvature in the range from 0.4D to 10.0D, where D is the diameter of the branch pipe 13. A high pressure fuel main pipe has tapered bearing surfaces 12'. A lock nut is tightened to secure the branch pipe to the main pipe while the spigot head is abuttingly engaged with each of the bearing surfaces 12'.
The conventional joint structure is satisfactory in the sense that the branch pipe 13 can readily be joined by tightening the lock nut, and if the spigot head 13' of the branch pipe 13 is offset, the spigot head 13' can automatically be aligned and held in linear contact with the spherical bearing surface 12' to insure the connection under even contact pressure as the lock nut is being tightened. However, the straight portion 14 lacks rigidity and is thus subject to deformation or inclination due to repeated supply of fuel under substantially high pressure and vibration. If this occurs, the front end of the passage 13" within the branch pipe 13 is collapsed to reduce its effective cross sectional area and increase the resistance to fuel flow. This blocks precise control of fuel injection. Moreover, the connection can not be maintained over a prolonged period of time due to unstable "seating" of the spigot head and loosening of the lock nut. This results in leakage of the fuel.